Conventional current source rectifiers assume operation in continuous mode where current flows through the direct current (“DC”) link inductor of the current source rectifier. In order to maintain continuous conduction of current through such a DC link inductor, either a large inductor has to be used, which negatively affects the size and power density of the current source rectifier, or the input current has to be maintained at a sufficiently high switching frequency, which negatively affects the efficiency of the current source rectifier. For this reason, voltage source rectifiers have been preferred over current source rectifiers for many applications that require small size or lower switching frequencies. However, voltage source rectifiers experience numerous problems that cause unwanted distortion on and conducted electromagnetic emissions into the input voltage and current sources. These problems can be mitigated using additional input filtering hardware, but this leads to additional size and thus even lower power density.
The current source rectifier is an attractive alternative to the voltage source rectifier, more widely used by industry, because it can achieve AC-to-DC voltage conversion with nearly sinusoidal input currents with less input filter components as well as with a smaller overall size. Current source rectifiers depend upon a continuous DC link current in order to convert alternating current (“AC”) input voltages to DC output voltages. That is, conventional current source rectifiers are built upon the assumption that the DC link current, such as ip shown in FIG. 1A, does not fall below zero during operation.
This is accomplished by keeping the inductance value or the input switching frequency and the average load current at sufficiently high values such that ip appears as a constant current source. The switching frequency and inductance are also typically selected such that the voltage ripple on ip is less than 10% of its rated value. But at a sufficiently light load values, the current source rectifier will enter discontinuous conduction mode of operation. The load at which discontinuous mode occurs will be more significant and more prevalent as the size of the DC link inductor Lp is reduced to improve power density or when the switching frequency is reduced to improve efficiency. Thus, conventional current source rectifiers simply attempt to avoid discontinuous operation by increasing the size of the DC link inductor Lp (thus reducing power density) or increasing the input switching frequency (thus reducing efficiency); or by adding additional hardware, such as dynamic braking, in order to avoid discontinuous operation altogether. These measures often make the current source rectifier an unattractive alternative for many applications.
FIG. 1A depicts an example block diagram of a conventional controller for a current source rectifier according to the prior art. The current source rectifier 110 shown in FIG. 1A is an active AC-to-DC rectifier that converts three-phase AC input power to a controlled DC voltage through an active rectification process and drives a constant power load 104. In the illustration, the three-phase AC input power is shown coming off of an AC power grid 102 and is provided to the current source rectifier 110 through input lines a, b, and c. A conventional controller 101 receives the input voltages via signal drivers 106 and 108. The conventional controller 101 also receives output voltage vo as well as the DC link current ip through the inductor LP from the current source rectifier 110. Controller 101 then provides control signals Ui1-Ui6 to control the switches Si1-Si6 of the current source rectifier 110, respectively.
FIG. 1B depicts an example block diagram of some of the main components of a conventional controller for a current source rectifier. As shown, conventional controller 101 includes feed-forward controls 120 and feedback controls 124. These units receive the output voltage vo as well as user selection of a desired output voltage vo*. The feedback controls 124 provide the modulation index mi to the duty cycle calculation unit 130. The duty cycle calculation unit 130 also receives the phase value φ of the three-phase input power and calculates the duty cycles dk—CCM and dn—CCM based on those values. The calculated duty cycles control the signal pulses dk that are provided to the current source rectifier to control the switches Si1-Si6 of the current source rectifier.
As used herein, the acronym “CCM” stands for “continuous conduction mode” and represents the fact that, for conventional current source rectifiers, it is assumed that the current through the DC link inductor is continuous during operation. When discontinuous conduction mode (“DCM”) occurs in a conventional current source rectifier, the resultant input currents exhibit low order harmonic distortion and a voltage boosting effect occurs on the output such that the output voltage of the current source rectifier is difficult to control. Even worse, when discontinuous conduction occurs in conventional current source rectifiers, it can also cause voltage stresses to occur on power semiconductor devices that can cause damage. These phenomena are graphically demonstrated in FIG. 2, which depicts example graphs showing characteristics of a conventional current source rectifier during discontinuous conduction mode of operation. The output voltage graph 201 demonstrates a loss of control of the output voltage vo. At startup, the output voltage vo far exceeds the user-selected output voltage vo*. Further, the input current graphs 204 demonstrate high distortion on the average input current <ik>, which is shown by the clipping pattern where the input signal should be substantially sinusoidal. The input currents are referred to herein generally as <ik> and <in>, which represent the input current on any two of the three input phases ia, ib or ic. These undesirable side-effects are caused during discontinuous conduction mode when at least part of the DC link current ip falls below zero as shown in graphs 206.
These undesirable side-effects are usually mitigated by dissipative circuitry to keep the current source rectifier from going too deeply into discontinuous conduction mode. Unfortunately this further increases the size of the circuit and reduces efficiency. Emerging low power applications (e.g., <10 kW) use Silicon Carbide (“SiC”) metal-oxide semiconductor field-effect transistors (“MOSFETs”) in current source rectifiers in order to allow very high switching frequency and reasonable efficiency. But SiC MOSFETs are expensive and have limited availability in size and range.